The invention relates to the making of lithographic printing plates and more particularly to an attachment for a graphic arts camera for use in making lithographic printing plates using electrophotographic techniques.
Offset plates are made photographically by complex methods requiring the use of silver halide film, photoresists, and the processing of the images to achieve their incorporation onto plates that can be used on printing presses. In recent years electrostatic techniques have also been used to make such plates, but their quality is so inferior to the quality of photographically made plates that this type of technique is utilized rarely.
The invention herein revolves around a type of electrophotographic film that is capable of being imaged with quality and gray scale as good as, if not better than, that achieved by photographic techniques and with resolution that is better than that achieved by photographic techniques. The film includes a photoconductive coating that is bonded onto a conductive substrate. In a preferred form the photoconductive coating comprises a layer of radio frequency sputtered cadmium sulfide that is from about 2,000 Angstroms to about 2 microns thick and the conductive substrate comprises a layer of about 300 to 500 Angstroms thick of indium tin oxide which in turn is disposed on a sheet of stable polyester plastic about 5 mils thick. The film is described in U.S. Pat. No. 4,025,339 issued May 24, 1977.
The film may be exposed at a high speed toned with an ink-receptive toner and the toned image so formed then transferred immediately to a blank lithographic printing plate which enables the entire process to be carried out inside of an attachment or cartridge which takes the place of the usual silver halide photographic film cartridge that is currently used with graphic art cameras. Besides the elimination of the silver halide film this eliminates considerable processing since the plate with the transferred, toned image may subsequently be fixed and then directly immersed in a suitable bath to render the untoned parts ink repellent and then utilized directly as a printing plate in an offset printing press.